Downdraft Fixed-Bed Gasifier for Producing a Product Gas from Pourable Biomass Particles

ABSTRACT

A downdraft fixed-bed gasifier for producing product gas from pourable biomass particles includes a gasifier container, a gasifier component, a feeder, an air supply inlet, a grate and a product gas vent. The gasifier container has a larger diameter than does the gasifier component. The lower open end of the gasifier component extends down into the gasifier container. The feeder is adapted to receive biomass particles into the upper closed end of the gasifier component. Combustion air is fed into the gasifier component through the air supply inlet located near the upper closed end. The grate supports the biomass particles and is disposed in a lower portion of the gasifier container below the lower open end of the gasifier component. Product gas generated from oxidizing the biomass particles exits the gasifier container through the product gas vent located in the side of the container below the level of the grate.

CROSS REFERENCE TO RELATED APPLICATION

This application is filed under 35 U.S.C. §111(a) and is based on andhereby claims priority under 35 U.S.C. §120 and §365(c) fromInternational Application No. PCT/EP2015/078888, filed on Dec. 7, 2015,and published as WO 2016/091835 A1 on Jun. 16, 2016, which in turnclaims priority from German Application No. 102014225166.4, filed inGermany on Dec. 8, 2014. This application is a continuation-in-part ofInternational Application No. PCT/EP2015/078888, which is a continuationof German Application No. 102014225166.4. International Application No.PCT/EP2015/078888 is pending as of the filing date of this application,and the United States is an elected state in International ApplicationNo. PCT/EP2015/078888. This application claims the benefit under 35U.S.C. §119 from German Application No. 102014225166.4. The disclosureof each of the foregoing documents is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a downdraft fixed-bed gasifier for producing aproduct gas from pourable biomass particles and to a method forstarting, operating and shutting down such a downdraft fixed-bedgasifier.

BACKGROUND

Fixed-bed gasifiers that generate a combustible product gas from biomasspellets, such as wood chips or wood pellets, are characterized by acomparatively simple design. A distinction exists between countercurrentgasifiers and downdraft gasifiers. In a countercurrent gasifier, thecombustion air and the product gas flow in a direction opposed to thefeed-in direction of the biomass particles. In a downdraft gasifier,however, the feed-in direction of the biomass particles matches the flowdirection of combustion air and product gas. Fixed-bed gasifiers havedifferent reaction zones, such as a drying zone, a pyrolysis zone, anoxidation zone and a reduction zone, in which different thermochemicalreactions take place.

An overview on the subject of fixed bed gasification of biomassparticles was disclosed by Lettner, Haselbacher and Timmerer from theTechnical University of Graz, Austria, in the presentation entitled“Festbett-Vergasung—Stand der Technik (Überblick)” (an overview of thestate of the art of fixed bed gasification) given on Feb. 27, 2007 atthe conference in Leipzig entitled “Thermo-chemische Biomasse-Vergasungfür eine effiziente Strom/Kraftstoffbereitstellung—Erkenntnisstand 2007”(thermo-chemical biomass gasification for efficient current/fuelsupply—state of the art in 2007). The presentation describes a downdraftshaft gasifier in which the biomass particles are fed into the gasifiercontainer from above using gravity. In the middle area of the gasifier,combustion air is supplied via nozzles and the product gas is dischargedfrom the lower area of the gasifier container. A drying zone, apyrolysis zone, an oxidation zone and a reduction zone are arranged fromtop to bottom in this known fixed-bed gasifier. The oxidation zone islocated within the area of the air supply and is to be restricted tothat zone. The reduction zone is beneath the oxidation zone and isdirectly above the grate. The product gas is removed from the area ofthe gasifier container beneath the grate, through which small particlesof ashes fall and are collected.

In order to maintain stable process control, the different zones atwhich the different thermochemical reactions take place should bemaintained at stationary positions in the gasifier container. Indowndraft gasifiers, the location of the oxidation zone is determined bythe location of the air supply through the nozzles. An air supply fromthe nozzles has the disadvantage that within the area of the oxidationzone a homogenous distribution of air does not occur and temperaturedifferences of up to 400° may occur locally. This may lead to depositsof combustion residues (slag) in undesired locations in the gasifierchamber, which impairs movement of the biomass particles and causesinhomogeneous gas flow that leads to increased tar quantities in theproduct gas.

Based on the downdraft fixed-bed gasifier according to theaforementioned presentation entitled “Festbett-Vergasung—Stand derTechnik (Überblick)” (an overview of the state of the art of fixed bedgasification), it is an object of the present invention to provide adowndraft fixed-bed gasifier, and a method for operating such adowndraft fixed-bed gasifier, in which harmful temperature gradientswithin the area of the oxidation zone are prevented. Furthermore, it isan object of the invention to provide a method for starting and shuttingdown such a downdraft fixed-bed gasifier.

SUMMARY

The invention relates to a downdraft fixed-bed gasifier for producing aproduct gas from pourable biomass particles and to methods for starting,operating and shutting down such a downdraft fixed-bed gasifier. Bysupplying air through a bed of biomass particles in a tubular gasifiercomponent, a uniform distribution of combustion air is achieved. Hardlyany temperature differences occur in the oxidation zone of the biomassparticles by virtue of the uniform air distribution. As a result, evenpyrolysis gases produced above the oxidation zone flow through theoxidation zone in a uniform manner. The uniformity of the flow of gasand air allows a product gas to be generated with low tar quantities.The oxidation zone is disposed both above and below a cross-sectionaljump at the open end of a gasifier component between the smallercylindrical gasifier component and the larger cylindrical gasifiercontainer. Different flow speeds of air and gas result from thecross-sectional jump from the smaller gasifier component to the largergasifier container. By expanding the cross-section through which the airand gas flow as the cross-sectional jump, the flow speed is slowedcompared to that of a conventional fixed-bed gasifier. The differentflow speeds within and outside the tubular gasifier component fix theoxidation zone in front of the lower open end of the tubular gasifiercomponent.

Another advantage of the expanding cross-section at the lower open endof the tubular gasifier component is that the pyrolysis gases are notconfined by the tube wall of the gasifier component as they flow throughthe oxidation zone. The flow conditions on tube walls are not uniform,and thus the temperatures there would not be uniformly high. Whenpyrolysis gases flow on the edge of a tube wall through the oxidationzone, as is the case in the prior art, the long-chain hydrocarbons arenot completely broken down. More long-chain hydrocarbon compounds arebroken down in the novel downdraft fixed-bed gasifier by virtue of theabsence of tube walls in the oxidation zone, thus leading to animprovement in the efficiency of any gas engine that operates using theproduct gas.

A downdraft fixed-bed gasifier for producing a product gas from pourablebiomass particles includes a gasifier container, a gasifier component, afeeder, a level sensor, an air supply inlet, a grate and a product gasvent. The cylindrical gasifier container has a larger diameter than thatof the cylindrical gasifier component. The gasifier component isarranged coaxially with respect to the gasifier container. The loweropen end of the gasifier component extends down into the gasifiercontainer so that the upper closed end of the gasifier componentsprojects up and out of the gasifier container. The grate is adapted tosupport the biomass particles and is disposed in a lower portion of thegasifier container below the lower open end of the gasifier component.The distance between lower open end of the gasifier component and thegrate is 90% to 110% of the diameter of the gasifier component. Thediameter of the gasifier component is 50% to 80% of the diameter of thegasifier container.

The feeder is adapted to receive biomass particles, such as woodpellets, into the upper closed end of the gasifier component. The levelsensor detects the level of the upper extent of the biomass particles inthe gasifier component. Combustion air is fed into the gasifiercomponent through the air supply inlet located near the upper closedend. A ceramic nozzle of an ignition device leads into the gasifiercontainer above the grate and below the lower open end of the gasifiercomponent. Hot air is injected into the gasifier container through theceramic nozzle and ignites the biomass particles. Product gas generatedfrom oxidizing the biomass particles exits the gasifier containerthrough the product gas vent located in the side of the container belowthe level of the grate. Ash is produced while product gas is generatedfrom the biomass particles. The ash is discharged from the gasifiercontainer in the product gas that exits the gasifier container throughthe product gas vent.

The downdraft fixed-bed gasifier also includes a biomass particlestorage container and a biomass particle conveyor. The biomass particleconveyor is connected to the feeder by a first airtight lock valve. Thebiomass particle storage container has a second airtight lock valve. Thebiomass particle conveyor moves biomass particles from the biomassparticle storage container through the feeder and into the gasifiercomponent in an airtight manner sealed from the outside environment.

A method of operating a downdraft fixed-bed gasifier includes variousstep leading to the generation of product gas from biomass particles. Agasifier component is filled with biomass particles to a predeterminedfill level. The biomass particles can be wood pellets with a 1%-5%portion of kaolin (hydrated aluminum silicate) by weight. A lower openend of the gasifier component extends down into a gasifier container.The gasifier component is arranged coaxially with respect to thegasifier container. The biomass particles are supported by a gratedisposed below the lower open end of the gasifier component in a lowerportion of the gasifier container. The grate is rotated. Combustion airis supplied through an air supply inlet into the gasifier component.When the downdraft fixed-bed gasifier is first started, the biomassparticles are ignited by injecting hot air with a temperature of morethan 300° C. into the gasifier container below the lower open end of thegasifier component. The product gas is discharged from a product gasvent leading out of the gasifier container below the level of the grate.

Additional biomass particles are added to the gasifier component so asto maintain the predetermined fill level of biomass particles in thegasifier component as biomass particles are consumed through thegeneration of product gas. Ash produced while the product gas is beinggenerated from the biomass particles is discharged from the gasifiercontainer in the product gas that exits the gasifier container throughthe product gas vent.

The downdraft fixed-bed gasifier is shut down by stopping the additionof additional biomass particles. Then the supply of air is stopped whenthe biomass particles have been consumed such that the upper extent ofthe biomass particles has dropped to a predetermined lower cutoff levelin the gasifier component.

Other embodiments and advantages are described in the detaileddescription below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a schematic, cross-sectional diagram of an exemplaryembodiment of the invention that includes the essential components.

FIG. 2 is a schematic representation of a combination of the downdraftfixed-bed gasifier of FIG. 1 together with a gas processing device and acombined heat and power unit (CHP).

FIG. 3 shows a bed of biomass particles in the gasifier of FIG. 1 andthe different reaction zones.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a schematic diagram of an exemplary embodiment of a downdraftfixed-bed gasifier 10. The downdraft fixed-bed gasifier 10 includes acylindrical gasifier component 11 inside a cylindrical gasifiercontainer 12. By supplying air 13 through a bed of biomass particles 14in the tubular gasifier component 11, a uniform distribution of air isachieved throughout the biomass particles. By virtue of the uniformdistribution of combustion air 13, hardly any temperature differencesoccur within the oxidation zone 15 of the gasifier container 12. As aresult, even pyrolysis gases produced above the oxidation zone 15 flowthrough the oxidation zone in a uniform manner. The uniformity of theflow of air and gases allows a product gas 16 to be generated with lowtar quantities.

By supplying air 13 from the top and by discharging product gas 16 fromthe lower part beneath a rotatable grate 17, air and gas flows throughthe fixed-bed gasifier 10 only from top to bottom. A drying zone 18 anda pyrolysis zone 19 are located in the gasifier component 11 whichprojects into the gasifier container 12. The oxidation zone 15 issituated beneath the open end 20 of the gasifier component 11, followedby the reduction zone 21 above the grate 17. The oxidation zone 15 islocally connected by gas flowing from top to bottom and by means of across-sectional jump from the gasifier component 11 outwards in thegasifier container 12 at the open end 20 of the gasifier component 11.Different flow speeds of the air and gas result from thiscross-sectional jump. By expanding the cross-section, the flow speed ofthe air is slowed compared to conventional fixed-bed gasifiers.Conventional fixed-bed gasifiers have a necking beneath the oxidationzone 15, which increases the flow speed of the gas. In the fixed-bedgasifier 10, the different flow speeds within and outside the tubulargasifier component 11 virtually fix the oxidation zone 15 in front ofthe open end 20 of the tubular gasifier component 11.

Another advantage of the expanding cross-section is that the pyrolysisgases generated in the pyrolysis zone 19 above the oxidation zone 15 arenot confined by a tube wall while flowing through the oxidation zone 15.The flow conditions over tube walls are not uniform, and thus thetemperatures there would not be uniformly high. When pyrolysis gas flowson the edge of a tube wall through the oxidation zone 15, as is the casein the prior art, the long-chain hydrocarbons are not completely brokendown. Additional long-chain hydrocarbon compounds are broken down byvirtue of the absence of the tube wall in the oxidation zone 15 beneaththe open end 20 of the gasifier component 11, thus leading to animprovement of efficiency when using the product gas 16 to power anengine 22.

The circular cross section and constant diameter 23 of the gasifiercomponent 11 allows the combustion air 13 and product gas 16 to flowuniformly and to produce an oxidation zone 15 with a homogeneoustemperature. A large portion of the long-chain hydrocarbon compounds arebroken down in the oxidation zone 15, thereby generating high qualityproduct gas 16.

By rotating the grate 17, clogging of the grate 17 is prevented andresidual agglomeration is reduced. In addition, rotating the grate 17allows ash to be removed from the gasifier container 12 as ash fallsthrough the grate and is carried as particles by the product gas 16 outof the product gas vent 24.

The optimum distance 25 from the grate 17 to the open bottom end 20 ofthe gasifier component 11 was determined empirically. The distance 25from the open end 20 of the gasifier component 11 to the grate 17approximately equals the diameter 23 of the gasifier component 11. Ifthe distance 25 is smaller than the optimum, the size of the reductionzone 21 is reduced, which negatively affects the quality of the productgas 16. If the distance 25 is larger than the optimum, the size of thereduction zone 21 increases, which likewise unfavorably affects to thequality of the product gas 16. Although the downdraft fixed-bed gasifier10 even functions at a deviation of 40% (h=d+/−40%) from the optimumdistance, the quality and yield of the product gas 16 is impaired.

The smaller the inner diameter 23 of the gasifier component 11 comparedto the inner diameter 26 of the gasifier container 12, the larger is thedifference of the gas flow speed within and outside the tubular gasifiercomponent 11. If the difference in gas flow speed is too large, thematerial efficiency is reduced and more fuel is required to generate thesame amount of product gas 16. If the difference in speed becomes toosmall, the gas flow speed exiting the gasifier component 11 is too high.In addition, the inner diameter 23 of the tubular gasifier component 11must be large enough to form a bed of biomass particles 14 in thegasifier component 11. The optimum relative size of the inner diameter26 of the gasifier container 12 compared to the inner diameter 23 of thegasifier component 11 was found empirically and results in a functionaldowndraft fixed-bed gasifier.

The biomass particle conveyor system 27 has airtight locks used forfeeding biomass particles 14 into the tubular gasifier component 11. Thebiomass particle conveyor system 27 may also be used in other fixed-bedgasifiers independent of the present invention.

The wood gas or other product gas 16 generated in the downdraftfixed-bed gasifier 10 is preferably used in a combined heat and powerunit (CHP) with a combustion engine 22 or a fuel cell for providingelectrical and thermal power. The product gas 16 generated in thedowndraft fixed-bed gasifier 10 is cooled and purified in a downstreamgas processing device 28.

The cooled and purified product gas 16 is mixed with combustion air 13in the gas mixing duct 29 of the combustion engine 22. Compared to theproduct gas 16 from the gas processing device 28, the combustion air 13is cold, and the product gas 16 is further cooled. Such further coolingmay lead to the undesired precipitation of solids or liquids andparticularly of tar. By providing a condensate separator 30 after thecombustion air 13 is added to the product gas 16 or wood gas butdirectly prior to combustion in the gas engine 22, the solid and/orliquid precipitations are deposited and separated from the product 16and thus cannot harm the gas engine 22. This configuration of gas mixingduct 29, condensate separator 30 and gas engine 22 can also be used inother types of fixed-bed gasifiers independent of the present invention.The gas processing device 28 includes a heat exchanger for cooling theproduct gas 16 to a temperature that can be used as fuel in a combinedheat and power (CHP) station.

A control device controls the biomass particle conveyor system 27, and alevel sensor 31 detects a fill level 31 of biomass particles 14 in thetubular gasifier component 11. The control device and level sensor 31ensure that the bed of biomass particles 14 in the gasifier component 11is high enough to sufficiently distribute the air 13 flowing through thebed of biomass particles 14 in a uniform manner over the entirecross-section of the gasifier component 11 before the air reaches thefirst reaction zone 19. By continuously supplying combustion air 13 overthe bed of biomass particles 14 and due to a continuous removal ofproduct gas 16, the different reaction zones of the downdraft fixed-bedgasifier 10 remain stationary, and predefined reaction conditions aremaintained.

By adding kaolin to the biomass pellets 14 in their manufacturingprocess, the melting point of the ash that results from the biomassgasification is increased and the likelihood that the grate 17 becomesclogged is reduced. Independent of the present invention, using suchpellets 14 with kaolin may also be used in an advantageous manner inother types of wood gasifiers.

The downdraft fixed-bed gasifier 10 is started by first filling thegasifier component 11 with biomass particles 14 up to a predeterminedtarget level 32 and then igniting the biomass particles 14 by blowing inhot air with a temperature of more than 300° C. into the bed of biomassparticles 14 beneath the open end 20 of the tubular gasifier component11. The temperature of the hot air is selected so that the biomassparticles 14 are ignited safely.

The operation of the downdraft fixed-bed gasifier 10 is stopped byterminating the supply of biomass particles 14, continuing to supplycombustion air 13 for a predetermined period of time or until the levelof biomass particles 14 has dropped to a preset lower cutoff level, andthen terminating the supply of combustion air 13. In the terminationprocess, biomass gasification continues after the biomass particles 14are no longer fed into the gasifier component 11, the level of the bedof biomass particles 14 in the gasifier component 11 drops, and aslittle as possible fresh biomass particles 14 remain. If after thesupply of air 13 has ended, a large portion of fresh biomass particles14 were to remain in the tubular gasifier component 11, the particles 14would outgas humid gas that would cause the overlying biomass particles14 to swell. Upon re-starting the gasifier 10, such swelling could leadto the gasifier component 11 becoming clogged.

FIG. 1 is a schematic diagram of an exemplary embodiment of a downdraftfixed-bed gasifier 10. The downdraft fixed-bed gasifier 10 includes atubular gasifier container 12, the ends of which are closed by an uppercover 33 and a lower cover 34. The tubular gasifier component 11 has alower open end 20 and a closed upper end 35 and projects with its openend 20 down into the gasifier container 12. The closed upper end 35 ofthe gasifier component 11 protrudes from the gasifier container 12through the upper cover 33. The open end 20 of the gasifier component 11lies approximately at mid height in the gasifier container 12. Therotatable grate 17 is positioned at a distance 25 below the open end 20of the gasifier component 11. The rotatable grate 17 can be periodicallymoved by a motor drive 36 whose drive shaft penetrates the lower cover34. A feeder 37, an air supply inlet 38 and the probe of the levelsensor 31 all lead into the gasifier component 11 near its closed end35. The biomass particles 14 are poured into the gasifier component 11through the feeder 37. Combustion air 13 flows into the gasifiercomponent 11 through the air supply inlet 38. And the level sensor 31monitors the level of the biomass particles 14 in the tubular gasifiercomponent 11.

An ignition device 39 and a closed inspection shaft 40 penetrate theouter wall of the gasifier container 12 and are disposed near the heightof the open end 20 of the gasifier component 11. The ignition device 39generates hot air with a temperature in a range of 300° C. to 600° C.,which is used ignite the biomass particles 14 in the area beneath theopen end 20 of the gasifier component 11 when starting the downdraftfixed-bed gasifier 10. The area beneath the open end 20 of the gasifiercomponent 11 is within the oxidation zone 15. The ignition device 39includes an air nozzle 41 for injecting hot air that aerates thegasifier container 12. By making the air nozzle 41 from ceramics, thoseparts of the ignition device 39 that are disposed within the gasifiercontainer 12 are thermally decoupled from the parts outside the gasifiercontainer 12. Maintenance and cleaning work can be performed inside thereactor vessel of the fixed-bed gasifier 10 during standstill of thereactor through the inspection shaft 40.

Product gas 16 is removed from the gasifier container 12 through theproduct gas vent 24 located below the grate 17. The ashes fallingthrough the grate 17 are discharged from the fixed-bed gasifier 10 bybeing swept up in the flow of product gas 16 out the product gas vent24.

Both the tubular gasifier container 12 and the tubular gasifiercomponent 11 have a circular and ring-shaped cross-section and arearranged concentrically to one another. The tubular gasifier component11 has an inner diameter 23 that is smaller than the inner diameter 26of the tubular gasifier container 12.

The feeder 37 for biomass particles 14 is connected to a biomassparticle conveyor system 27 through a first lock valve 42 on top of thefeeder 37 and a screw conveyor 43. The screw conveyor 43 is connected toa biomass particle storage container 44 in a airtight manner. Thestorage container 44 is loaded with biomass particles 14 through asecond lock valve 45 that seals the storage container 44 airtight fromthe outside environment. Because the biomass particle storage container44 is directly connected in an airtight manner to the screw conveyor 43,those two components maintain the air lock between the two lock valves42 and 45 as the biomass particles 14 are fed into the fixed-bedgasifier 10.

FIG. 2 illustrates a combination of the fixed-bed gasifier 10 of FIG. 1with a downstream gas processing device 28 and a combined heat and powerunit (CHP) 46. The product gas 16 escaping from the product gas vent 24is supplied to the gas processing device 28. The product gas 16 iscooled in a heat exchanger inside the gas processing device 28, and bothsolid and liquid impurities are separated to the extent possible.

The cooled and processed product gas 16 from the product gas processingdevice 28 is supplied via a product gas line 47 to a gas mixing duct 29of a gas engine 22. The gas mixing duct 29 is connected to a combustionair supply manifold 48. By mixing comparatively cold external air andcomparatively hot product gas 16 from the product gas processing device28 in a mixed gas line 49, the resulting gas mixture is further cooledso that further liquid impurities precipitate out. The liquid impuritiesare deposited in a condensate separator 30 at the end of the mixed gasline 49 directly in front of the supply of the gas mixture to the gasengine 22 and can be conveniently removed. The quality of the mixed gasis thereby enhanced, and harmful impurities are prevented from beingburned in and emitted from the gas engine 22.

FIG. 3 shows the downdraft fixed-bed gasifier 10 in normal operatingconditions with the bed of biomass particles 14 in the gasifiercomponent 11 as well as in the gasifier container 12 below the gasifiercomponent 11. FIG. 3 also shows the positions of the different reactionzones. The oxidation zone 15 is disposed directly below the open end 20of the gasifier component 11. Beneath the oxidation zone 15 is thereduction zone 21 that extends down to the grate 17. The pyrolysis zone19 extends up from the oxidation zone 15 and is followed by the dryingzone 18.

In order to start the downdraft fixed-bed gasifier 10, the gasifiercontainer 12 is first filled with biomass particles 14 up to a targetlevel 32 via the feeder 37 so that the bed of biomass particles 14 isformed. By blowing in hot air from the ignition device 39, the biomassparticles 14 are ignited directly below the open end 20 of the gasifiercomponent 11 and form the oxidation zone 15. As soon as combustion ofbiomass particles 14 within the oxidation zone 15 is self-sustaining,the ignition device 39 is shut off. Beginning before the ignition of thebiomass particles 14, combustion air 13 is supplied into the gasifiercomponent 11 through the air supply inlet 38. The combustion heatreleased in the oxidation zone 15 gradually forms the remaining reactionzones. The product gas 16 flowing out the product gas vent 24 issupplied to the gas processing device 28. The quality of the product gas16 is monitored in the gas processing device 28. During startup of thedowndraft fixed-bed gasifier 10, the flow of product gas 16 from theproduct gas vent 24 is burned in an exhaust gas torch (not shown) onaccount of its poor quality. As soon as a sufficient quality of theproduct gas is reached, the product gas is cooled in the gas processingdevice 28 and is separated, to the extent possible, from solid andliquid impurities. During normal operating conditions of the fixed-bedgasifier 10, the target level 32 of the biomass particles 14 in thegasifier component 11 is monitored by level sensor 31 and, if necessary,biomass particles 14 from the biomass particle conveyor system 27 arerefilled through the first lock valve 42 and the feeder 37 in order tofill the gasifier component 11 back to the target level 32.

Upon shutting down the fixed-bed gasifier 10, the resupply of biomassparticles 14 is first stopped so that the level of biomass particles 14in the gasifier component 11 drops below the target level 32. If a largeamount of fresh biomass particles 14 were to remain in the tubulargasifier component 11 after the supply of combustion air 13 is shut off,the biomass particles 14 would outgas and the humid gas would cause theoverlying biomass particles 14 to swell. Upon re-starting the gasifier,such swelling could then lead to the tubular gasifier component 11becoming clogged. When the biomass particles are consumed such that theupper extent of the bed of particles drops down to a lower cutoff level50 in the gasifier component 11, the air supply is shut off, and the gasproduction terminates. The lower cutoff level 50 correspondsapproximately to the upper extent of the pyrolysis zone 19 during normaloperating conditions. In this way, a small amount as possible of freshbiomass particles 14 remains in the gasifier container 12 and in thegasifier component 11 when the fixed-bed gasifier 10 is shut down.

The downdraft fixed-bed gasifier 10 is particularly suited for gasifyingwood pellets and biomass pellets. When the biomass pellets are prepared,kaolin (hydrated aluminum silicate) is added to the pellets so that thefinished pellets contain a mass proportion of kaolin of 1% to 5% andpreferably 1.5% to 3%. Through the addition of kaolin, the melting pointof the ash resulting from biomass gasification is increased so as toavoid the clogging of the grate 17 and any undesired deposit of ash atother components of the fixed-bed gasifier 10.

REFERENCE NUMERALS

10 down-draft fixed-bed gasifier

11 gasifier component

12 gasifier container

13 air

14 biomass particles

15 oxidation zone

16 product gas

17 grate

18 drying zone

19 pyrolysis zone

20 open end of gasifier component 11

21 reduction zone

22 gas engine

23 inner diameter of gasifier component 11

24 product gas vent

25 distance from grate 17 to open end 20

26 inner diameter of gasifier container 12

27 biomass particle conveyor system

28 gas processing device

29 gas mixing duct

30 condensate separator

31 level sensor for biomass particles

32 target fill level of biomass particles

33 upper cover

34 lower cover

35 closed end of gasifier component 11

36 motor drive for grate 17

37 feeder for biomass particles

38 air supply inlet

39 ignition device

40 inspection shaft

41 ceramic air nozzle

42 first lock valve

43 screw conveyor

44 biomass particle storage container

45 second lock valve

46 combined heat and power unit (CHP)

47 product gas line

48 combustion air supply manifold

49 mixed gas line

50 lower cutoff level of biomass particles upon shut-down

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

1-20. (canceled)
 21. A downdraft fixed-bed gasifier for producing aproduct gas from pourable biomass particles, comprising: a gasifiercontainer with a first diameter; a gasifier component with a seconddiameter, an upper closed end and a lower open end, wherein the loweropen end of the gasifier component extends down into the gasifiercontainer, wherein the upper closed end of the gasifier componentsprojects up and out of the gasifier container, and wherein the firstdiameter is larger than the second diameter; a feeder adapted to receivebiomass particles into the upper closed end of the gasifier component;an air supply inlet that enters the gasifier component near the upperclosed end and through which combustion air is fed into the gasifiercomponent; a grate adapted to support the biomass particles that isdisposed in a lower portion of the gasifier container; and a product gasvent leading out of the gasifier container below the grate and throughwhich the product gas generated from the biomass particles exits thegasifier container, and wherein the gasifier component is disposed inthe gasifier container such that a first distance remains between thelower open end of the gasifier component and the grate.
 22. Thedowndraft fixed-bed gasifier of claim 21, wherein each of the gasifiercontainer and the gasifier component has a circular cross-section. 23.The downdraft fixed-bed gasifier of claim 22, wherein the gasifiercomponent is arranged coaxially with respect to the gasifier container.24. The downdraft fixed-bed gasifier of claim 22, wherein each of thegasifier container and the gasifier component has a constant diameter.25. The downdraft fixed-bed gasifier of claim 21, wherein the grate isrotatable.
 26. The downdraft fixed-bed gasifier of claim 21, wherein thefirst distance is 90% to 110% of the second diameter.
 27. The downdraftfixed-bed gasifier of claim 21, wherein the second diameter is 50% to80% of the first diameter.
 28. The downdraft fixed-bed gasifier of claim21, further comprising: a nozzle of an ignition device leading into thegasifier container above the grate and below the lower open end of thegasifier component, wherein the nozzle is adapted to inject hot air intothe gasifier container.
 29. The downdraft fixed-bed gasifier of claim28, wherein the nozzle is ceramic.
 30. The downdraft fixed-bed gasifierof claim 21, further comprising: a biomass particle storage container;and a biomass particle conveyor, wherein the biomass particle conveyoris connected to the feeder by a first airtight lock valve, wherein thebiomass particle storage container has a second airtight lock valve, andwherein the biomass particle conveyor moves biomass particles from thebiomass particle storage container through the feeder and into thegasifier component in an airtight manner sealed from the outsideenvironment.
 31. The downdraft fixed-bed gasifier of claim 21, furthercomprising: a gas processing device connected to the product gas vent,wherein the gas processing device is connected to a combustion enginethat operates using the product gas.
 32. The downdraft fixed-bedgasifier of claim 31, wherein the gas processing device includes a heatexchanger that cools the product gas received from the product gas vent.33. The downdraft fixed-bed gasifier of claim 21, further comprising: alevel sensor that detects a level of the biomass particles in thegasifier component.
 34. A method of operating a downdraft fixed-bedgasifier, comprising: filling a gasifier component with biomassparticles to a predetermined fill level, wherein a lower open end of thegasifier component extends down into a gasifier container, wherein thegasifier component is arranged coaxially with respect to the gasifiercontainer, and wherein the biomass particles are supported by a gratedisposed below the lower open end of the gasifier component in a lowerportion of the gasifier container; supplying air through an air supplyinlet into the gasifier component; discharging a product gas from aproduct gas vent leading out of the gasifier container below the grate,wherein the product gas is generated from the biomass particles; andadding additional biomass particles to the gasifier component so as tomaintain the predetermined fill level of biomass particles in thegasifier component as biomass particles are consumed by generating theproduct gas.
 35. The method of claim 34, further comprising: rotatingthe grate.
 36. The method of claim 34, wherein the biomass particles arewood pellets.
 37. The method of claim 34, wherein the biomass particlesinclude a portion of kaolin, and wherein the portion is 1% to 5% byweight.
 38. The method of claim 34, further comprising: igniting thebiomass particles by injecting air with a temperature of more than 300°C. into the gasifier container below the lower open end of the gasifiercomponent.
 39. The method of claim 34, further comprising: stopping theadding of additional biomass particles; and stopping the supplying ofair when the biomass particles have been consumed such that an upperextent of the biomass particles has dropped to a predetermined lowercutoff level in the gasifier component.
 40. The method of claim 34,wherein ash is produced as product gas is generated from the biomassparticles, further comprising: discharging the ash from the gasifiercontainer in the product gas that is discharged from the product gasvent.